1. Field of the Invention
The present invention relates to Schiff base compounds and nonlinear optical materials comprising the Schiff base compounds, which are applicable to electro-optical devices, such as a second higher harmonic generation device, a piezoelectric device, an optical waveguide, a light source for an optical memory and a laser printer, and an optical switching device.
2. Discussion of the Background
In recent years, much attention is focused on materials having a nonlinear optical effect. When a laser beam having a relatively strong light intensity is applied to such materials, a light beam with a different light component from that of the applied laser beam is emitted therefrom. Such materials are conventionally known as nonlinear optical materials and described, for instance, in the following references: "Nonlinear Optical Properties of Organic and Polymeric Materials" ACS SYMPOSIUM SERIES 233, edited by David J. Williams (American Chemical Society, 1983), "Organic Nonlinear Optical Materials" compiled by Masao Kato and Hachiro Nakanishi (CMC Co., Ltd., 1983), and "Organic Electronics Materials" edited by Akio Taniguchi (Science Forum Co., Ltd., 1986).
The nonlinear optical materials are used in practice, for example, in a second higher harmonic generation (SHG) device, which utilizes a secondary nonlinear optical effect of the materials, and in a wavelength conversion device. Specific examples of the nonlinear optical materials which are employed in practice in the above devices are inorganic compounds such as potassium dihydrogenphosphate (KDP), ammonium dihydrogenphosphate (ADP) and lithium niobate. Recently it has been discovered that .pi.-electron-conjugated organic compounds having electron donative groups and electron attractive groups surpass the above-mentioned inorganic compounds in the nonlinear optical effect.
Generally, in the case of organic compounds, each molecule shows a nonlinear optical response and the nonlinear optical performance of each molecule depends upon the magnitude of the molecular hyperpolarizability (.beta.). However, many organic compounds, as represented by p-nitroaniline, have high second nonlinear optical performance, that is, high molecular hyperpolarizability (.beta.), in a gas state in which each molecule is separately and independently present, but never show the second nonlinear optical properties in a crystalline state, because in the crystalline state, such compounds have a centrosymmetric structure in the molecular arrangement thereof and therefore no molecular hyperpolarizability is exhibited.
As described in J. Appl. Phys. 50,2523 (1970) by B. F. Levine, et al., 2-methyl-4-nitroaniline (MNA), which is prepared by introducing a methyl group at an orthoposition of the above-mentioned p-nitroaniline, does not have such a centrosymmetrical structure as in p-nitroaniline since the centrosymmetrical structure is successfully destroyed by the introduction of a methyl group, but maintains the same high molecular hyperpolarizability (.beta.) as in p-nitroaniline, so that the thus obtained MNA has a large second higher harmonic generation tensor d.sub.11. However, the MNA does not satisfy the conditions under which second higher harmonic generation can be effectively obtained. Therefore, it is extremely difficult to effectively utilize the large nonlinear optical performance of the MNA.
In addition, it is extremely difficult to obtain the MNA in a single crystalline state, so that this compound is is not suitable for the material for use in nonlinear optical devices.
Furthermore, a method of obtaining a nonlinear optical medium by dispersing a compound with high nonlinear optical effect in a ploymeric material and subjecting the dispersion to poling under application of a magnetic field is disclosed in Japanese Laid-Open Patent Application 61-186942. However this method does not necessarily provide a satisfactory optical medium.